The present invention relates generally to bearings for use in deep well submersible pump systems, and more particularly to such bearings used to transmit radial loads and that are exposed to high temperature fluids being pumped by submersible pump systems.
Deep-well submersible (DWS) pumping systems (also referred to as electric submersible pumps (ESP)) are especially useful in extracting valuable resources such as oil, gas and water from deep well geological formations. In one particular operation, a DWS pump unit can be used to retrieve geothermal resources, such as hot water, from significant subterranean depths. In a conventional configuration, a generally centrifugal pump section and a motor section that powers the pump section are axially aligned with one another and oriented vertically in the well. More particularly, the motor section is situated at the lower end of the unit, and drives one or more pump section stages mounted above.
Because DWS pumping systems are relatively inaccessible (often completely submerged at distances between about 400 and 700 meters beneath the earth's surface), they must be able to run for extended periods without requiring maintenance. Such extended operating times are especially hard on the bearings that must absorb radial and axial forces of the rotor that is used to transmit power from the motor section to the impellers of the pump section. Radial bearings are one form of bearings employed in DWS systems, and are often spaced along the length of the rotor, particularly in a region where two axially adjacent rotor sections (such as between adjacent pump bowls in a serial multi-bowl assembly) are joined. These bearings are generally configured as sleeve-like sliding surfaces that are hydro dynamically lubricated between the surfaces by a contacting liquid. In one form, radial bearings in the pump section are situated in bowls that are lubricated by the fluid being pumped, while radial bearings in the motor section are lubricated by a coolant used to fill portions of the motor housing. For motors used in geothermal applications, the motor section lubricant is typically oil.
Conventional radial bearings for submersible DWS systems are not configured to withstand the high operating temperatures and pressures associated with the DWS environment, and as such have been prone to early failure. For example, in situations involving geothermal wells, the water being extracted from the earth may be 120 to 160 degrees Celsius or more, making the job of an on-board coolant (whether it be oil-based or water-based) all the more difficult. In addition, any impurities in the water that come in contact with the bearing surfaces of the pump section could leave deposits that may contribute to premature bearing wear or other operability problems. The problem is also particularly acute in the motor section, where radial bearing are generally not configured to guide or otherwise introduce sufficient motor cooling fluid into the bearing contact surface to promote adequate lubrication, especially at the elevated temperatures experienced inside the DWS motor section. That the hydrodynamic properties of the bearing need to be maintained not only in high temperature environments where the lubricating liquid has low viscosity, but also during start-up and shut-down phases of motor operation when the lubricating liquid generally is highly viscous (or not even present) exacerbates the design challenges. As such, there exists a desire for a bearing suitable for operation in deep well environments.